VIROLOGY Seasonal Collection of specimens and disease/epidemiological data all year round Diagnosis, virus isolation, preliminary analysis 2a
Virus isolation in eggs/cells
1-3 weeks 4a Serological studies 3-16 weeks Selection of candidate viruses for vaccine use 1 week 6a Classical
reassortment 3-4 weeks
7a
Antigenic and genetic
characterisation of reassortants
4 weeks 9a Vaccine
standardised reagent
development 6 weeks 1-3 weeks
Reverse engineering (and full safety testing)
6 weeks 6b 1 week Antisera production 3 3-5 weeks Antigenic and genetic anaylsis 1-3 weeks 4 5 hours – 3 weeks 2 1 H5N1
Growth property evaluation
Vaccine 8 3 weeks
standardised reagent
development 6 weeks 9b
Antigenic and genetic
characterisation of reassortants
4 weeks
7b
Availability of vaccine viruses and standardised reagents
Process of influenza vaccine virus selection and development. This diagram shows the individual steps in the selection of candidate vaccine viruses and development of standardising reagents for seasonal influenza and for a potential H5N1 influenza pandemic. For seasonal vaccines the timelines are: Steps 1–4: The collection, isolation and thorough antigenic and genetic characterisation of recent virus isolates continues throughout the year. Step 4a: Comparisons of the recognition of representative recent viruses by vaccine-induced antibodies in human and animal sera are conducted two-to-three weeks before the biannual WHO vaccine consultation meetings. Steps 5, 6a and 7a: Candidate viruses for vaccine use are reviewed and selected, and high-growth reassortants prepared and characterised following identification of potential antigenic variants. These steps are not solely dictated by the recommendations of the WHO biannual vaccine virus consultations, but may be used to evaluate several potential antigenic variants. Step 8: Evaluation of their growth properties is conducted in a timely manner around the time of the WHO vaccine virus consultations and prior to authorisation of vaccine composition by national authorities. Step 9a and 9b: Preparation of the standardising reagents for new vaccine components is initiated once the particular vaccine virus has been selected following the WHO recommendation.
considered for licensure in the territories where they are sold. The entire process requires coordination of activities, and evaluation and development of strains which can be used to deliver appropriate growth yield in different technology platforms (termed high growth strains).
This is a part of the process which is bespoke to each vaccine manufacturer.
Strain mismatch Every effort is made to ensure that surveillance information is as timely and accurate as it is possible to be at the
The necessity for frequent updates to vaccine
composition was first recognised in the 1940s. This led to the creation of an international surveillance network under the auspices of the WHO
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WWW.PATHOLOGYINPRACTICE.COM May 2026
time of vaccine strain selection. Given the complexity of the process and the timescales involved, it is unsurprising that there may occasionally be a mismatch between strains included in the vaccine and what is circulating. This is most likely to happen for an influenza A subtype as the rate of evolution is higher for influenza A than influenza B. Vaccine strains may be updated to improve yield or provide a beter genetic match to circulating strains, even in the absence of a significant drift variant, and there is a process of continual updating. It is considered most important to
have a good match for influenza A(H3N2) as this strain is widely recognised to cause more severe illness. A vaccine strain mismatch, due to the emergence of antigenic drift variants in H3N2, occurred
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